Using shin/elbow twist bones?
The previous answer is wrong.
The reason sometimes you see a bone or sometimes multiple bones in the middle of a limb section such as in between the shoulder and elbow bone, or in between the elbow and wrist bones is for volume preservation during twisting.
Do this:
Stick your arm forward palm down.
Now just turn your palm up but watch your forearm.
You will notice that your entire forearm twists smoothly across its length!
but if you turn your hand to the side or up and down, just the hand turns.
The idea of placing a bone in the middle of the forearm is to separate the twist from the other bone rotations for better skin weight deformations.
the same applies to the upper arm. if you stick your arm out to the side and twist your arm, you will see that your bicep muscle kind of sort of smoothly follows the twist of the arm WHILE the deltoid DOES NOT! so the idea of placing a joint in the middle of the bicep is, again, to separate the twisting from the other two rotations for better skin weight distribution.
If you can get away with more twist bones, the better as they will help preserve volume better than a single bone.
the same applies to the leg.
If you twist your leg, the big leg muscles near your hips will do little twisting and your gluts will do very little.
And same with your calf if you twist your foot HOWEVER the effect is much much smaller there.
If you want more help understanding this, look into the "Cane Toad" rig for Maya. Variations of it are kind of the standard across platforms.
I'm actually trying to find out if it's easy to setup something like this in blender before I decide to switch to it.